|  | 1.4 | Network Layer Functions
| 1.4.6 | Pin g command
Figure shows a router receiving a packet that it is unable to deliver to its ultimate destination; because of this the router sends an ICMP host unreachable message to the source. The message might be undeliverable because there is no known route to the destination. On the other hand, Figure shows an echo reply that is a successful reply to a ping command.